The present invention relates to non toxic gas generants which upon combustion, rapidly produce gas that is useful for inflating a vehicle airbag, and specifically the present invent relates to the process of grinding nitroguanidine, the fuel in the gas generant.
Vehicle airbag systems have been developed to protect a vehicle occupant in the event of a crash by rapidly inflating a cushion between the vehicle occupant and the interior of the vehicle. The gas for inflating the vehicle airbag is produced by a chemical reaction in an inflator. In order for an airbag to function properly, the airbag needs to be deployed within a fraction of a second.
For a pyrotechnic inflator, the gas production is a result of the combustion of a fuel inside the inflator. Both organic and inorganic fuels can be utilized for gas generants. Sodium azide, an example of an inorganic fuel, was the most widely used and accepted fuel for gas generants. The combustion of sodium azide occurs at a very rapid rate, which made it a suitable material for use as a gas generant. However, sodium azide has several inherent problems which has lead to extensive research on developing gas generants based on non-azide fuels. Sodium azide is a toxic starting material, since its toxicity level as measured by oral rat LD50 is in the range of 45 mg/kg. Another disadvantage of using sodium azide is that some of the combustion products can be toxic and corrosive. Recently, a new problem has surfaced concerning the disposal of unused airbag systems in cars at the end of their service life.
Because of the foregoing problems associated with sodium azide, the industry has developed many non-azide gas generants that are being used in some airbag inflators. One of the disadvantages of known non-azide gas generant compositions is the amount and physical nature of the solid residues formed during combustion. These solid combustion products must be filtered and kept away from contact with the vehicle occupants. It is therefore highly desirable to develop non-azide chemical compositions that have a higher gas conversion rate and produce essentially no slag or solid particles. Another disadvantage of using non-azide generants is that toxic side products of CO and NOx can be produced. The stoichiometric ratio and chemical structure of the reactants has a huge bearing on the levels of CO and NOx that are produced.
Many non-azide fuels have been researched that when mixed with the proper oxidizer produces little ash or slag during combustion and produce tolerable levels of toxic gas. Nitroguanidine is a fuel that when properly formulated possesses these desirable properties. Nitroguanidine is rich in nitrogen and burns very cleanly. The disadvantage of utilizing nitroguanidine is that when the fuel is compressed into a pellet, the pellet will grow or lose density when subjected to thermal cycling causing the ballistic properties to be altered.
U.S. Pat. No. 5,531,941 teaches a gas generant composition that has a very high gas yield and low yield of solid combustion products. One of the preferred gas generant composition consists of (a) about 59.4 wt. % of phase stabilized ammonium nitrate (b) about 32.48 wt. % of triaminoguanidine nitrate and (c) about 8.12 w % of guanidine nitrate.
U.S. Pat. No. 5,545,272 teaches a gas generating composition consisting of a mixture of nitroguanidine and phase stabilized ammonium nitrate. The patent does not address the influence of nitroguanidine on pellet size during thermal cycling.
U.S. Pat. No. 5,641,938 teaches a gas generating composition consisting of nitroguanidine, phase stabilized ammonium nitrate, and an elastomeric binder. The binder functions to control pellet growth.
U.S. Pat. No. 5,747,730 teaches a eutectic solution for a gas generant comprising ammonium nitrate, guanidine nitrate and/or aminoguanidine nitrate, and minor amounts of polyvinyl alcohol and either potassium nitrate or potassium perchlorate. The eutectic solution with the foregoing components will eliminate pellet cracking and substantially reduce ammonium nitrate phase change due to temperature cycling.
One aspect of the present invention is to grind nitroguanidine needles that will be used in a gas generant composition. When synthesized, nitroguanidine precipitates from solution as tough needles. Grinding or crumbling the nitroguanidine needles prevents the fuel from losing density during thermal cycling. The grinding converts the needle crystals to an amorphous crumb.
An advantage of the present invention is that the burn rate is increased because of increased particle size surface area. The burn rate for the preferred gas generant formulation is about 0.6 inches per second at 1000 psi.
Another advantage of the present invention is that it is not necessary to add a binder to stabilize the density of the gas generant containing nitroguanidine.